Since Foreign Keys can be nullable, support "None" in form field data

Right now the Ajax popup for foreign keys with foreign_key_lookups defined won't allow "None" to save. This adds support for "None" as a valid value.

Not sure if this is the way you would do it though.

I also made a change to flask-peewee to show "None" as an option in the list in the ajax popup that I can send a pull request for.